1. Field of the Invention
This invention relates to raster output scanners (ROS). More particularly it relates to an active element in the optical path of a ROS which addresses scan line skew and bow correction.
2. Description of the Prior Art
Electrophotographic marking is a well-known, commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a charged photoreceptor with a light image representation of a desired document. The photoreceptor is discharged where exposed, creating an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto that latent image, forming a toner image, which is then transferred onto a substrate, such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure, thereby creating a permanent record of the original representation. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for subsequent image production.
The foregoing broadly describes a black and white electrophotographic marking system. Electrophotographic marking can also produce color images by repeating the above process once for each color of toner that is used to make a composite color image. In one example of a color process, called a READ IOI process (Recharge, Expose, and Develop, Image On Image), a charged photoreceptive surface is exposed to a light image which represents a first color, say black. The resulting electrostatic latent image is then developed with black toner to produce a black toner image. The recharge, expose, and develop process is repeated, using the same photoreceptor, for a second color, say yellow, then for a third color, say magenta, and finally for a fourth color, say cyan. The various latent images and color toners are placed in a superimposed registration such that a desired composite color image results. That composite color image is then transferred and fused onto a substrate. Alternatively, a multiple exposure station system can be employed, requiring a separate charging, exposing, and developing station for each color of toner.
One way of exposing a photoreceptor in systems such as those described above is to use a Raster Output Scanner (ROS). A ROS is typically comprised of a laser light source (or sources), a pre-polygon optical system, a rotating polygon having a plurality of mirrored facets, and a post-polygon optical system. In a simplified description of operation, a collimated laser beam is reflected from the facets of the polygon and passed through imaging elements that project the laser beam into a finely focused spot of light on the photoreceptor's surface. As the polygon rotates (with the photoreceptor fixed), the focused spot traces a path on the photoreceptor surface referred to as a scan line. By synchronizing motion of the photoreceptor with the polygon rotation, the spot raster scans (i.e., one line after another) the surface of the photoreceptor. By modulating the laser beam with image information a predetermined latent image is produced on the photoreceptor. The plane of the sweeping beam is referred to herein as the tangential plane while the direction of motion of the photoreceptor is called the sagittal direction.
Raster output scanners are typically comprised of a number of optical elements. Unfortunately, unavoidable imprecision in the shape and/or mounting of these optical elements inevitably introduces anomalies in the quality of the scan line on the photoreceptor. One such anomaly is slight variation in scan line spacing on the photoreceptor. Such spacing variation, even if slight, can lead to perceptible tone variation in the scan line direction of the printed image, commonly referred to as banding artifacts. FIG. 1 shows light and dark streaks within an image 4 which represent banding artifacts in that image as printed.
Another such artifact is called bow. Bow is a deviation of a scan line in the shape of a frown or a smile. FIG. 2 illustrates two scan lines having different bows, a first scan line 6 has a “smile” shaped bow while the second scan line 7 has a “frown” shaped bow. FIG. 2 also shows an ideal scan line 5 without bow. A useful measurement for bow is the deviation between the top and the bottom of the scan line. In a monochromatic system if the bow deviation is kept below about 150 microns then the bow does not create a significant print quality problem. However, in color printing, particularly when using multiple raster output scanners, such errors seriously degrade print quality. Indeed, when multiple raster output scanners are used, if one bow forms a frown while the other forms a smile, bow errors of less than 10 microns degrade the final image. In high quality systems scan line bow should be held to about 2 microns.
Still another such artifact is called skew. While bow is a nonlinearity in the scan line, skew is an angular deviation of the scan line from the plane of the rotation axis of the photoreceptor. That is, tilt relative to the desired scan line. Artifacts such as line-to-line registration error, rotation of the printed image, etc. result.
One source of these artifacts is a slight imperfection in the advancement of the photoreceptor relative to the scan line. If this advancement is off by a slight amount, the scan line spacing from one line to the next varies slightly. In such a case, there may be a gap in the toner or similar material applied to the substrate as between two adjacent lines. This gaps affects the tone of the printed image along an entire scan line. The human eye is particularly sensitive to this type of artifact, and perceptible light and dark bands appear in the printed image.
Another source of such scan line spacing variations occurs when the center ray of a light beam passing through a lens does not scan along the optical axis of the lens. The farther the center ray of the beam is from the optical axis of the lens, the greater the bow. It should be noted that while it is the scan line deviations from the optical axes of the post polygon optical elements that usually produces bow, almost any optical component can introduce those deviations.
Various approaches to scan line position error correction exist today. One method is to use high quality optical systems, such systems being carefully matched when multiple raster output scanners are used. However, this approach is often prohibitively expensive, particularly when machine assembly is taken into consideration. Even then, meeting a 2 micron bow deviation requirement cannot always be met. Another approach is to add an optical element into the raster output scanner's optical system. For example, U.S. Pat. No. 5,383,047 teaches the introduction of a glass plate into the pre-polygon optical system. Rotation of that glass plate corrects for bow. However, that approach requires the introduction of another piece of glass into the optical path. Furthermore, in many raster output scanners it is the post-polygon optical system that introduces most of the scan line position error.
Other examples of known scan line position error correction use active feedback to adjust speed and line spacing, for example by using a piezo actuator to adjust in real time the position of the scan line. This is accomplished by actively translating a small lens element or tilting a mirror or in order to deflect the entire scan line. See, e.g., U.S. Pat. No. 6,232,991, which is incorporated herein by reference. However, any real time line spacing correction mechanism requires an actuator having a resonant frequency slightly above the line scan frequency, typically on the order of a few kHz. Consequently, printing speed is then limited by the resonant frequency of the feedback control system. Furthermore, bow and skew can lead to color registration or banding issues on part of a scan line even when feedback control systems are use because such systems measure scan positions at the edges of the photoreceptive drum or belt.
It should further be noted that the aforementioned line spacing, bow, and skew issues have heretofore prohibited seamlessly integrating side-by-side ROS systems in order to extend ROS-based printing systems to wider formats.
In light of the foregoing, and in further view of the desire to provide systems capable of printing without perceptible banding, bow, and skew, a new system and technique of correcting scan line position errors is needed.